1. Field of the Invention
This invention generally relates to capacitors, and more particularly, to capacitors that may be embedded within printed circuit boards or other microelectronic devices.
2. Background of the Invention
Capacitors are devices used for introducing capacitance into a circuit. Capacitors function primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. They comprise a layer of dielectric material sandwiched between a pair of spaced conductive metal layers, such as copper foils.
Capacitors are common elements on printed circuit boards (PCBs) and other microelectronic devices. In recent years, substantial efforts have been expended in the design of such PCBs and devices arranged thereupon to compensate for voltage fluctuations arising between the power and ground planes in the PCBs. One common type of voltage fluctuations include “switching noises,” which may be caused by switching operation of transistors in the integrated circuits. A common solution to this problem is to place one or more capacitors serving as a decoupling capacitors or bypass capacitors, which may be coupled between the power and ground terminals in proximity to the integrated circuits.
Capacitors may be electrically connected either as discrete elements on a circuit board, or may be embedded within the circuit boards. Of these options, forming embedded capacitors within the circuit boards allows increased surface area of the board for other purposes.
Two main factors for selection of a capacitor include the capacitance and the frequency bandwidth of a capacitor. The frequency bandwidth of a capacitor depends on its self-resonance frequency because a capacitor behaves properly when it operates in a frequency below the self-resonance frequency. Equation (1) below shows the relationship between capacitance and self-resonance frequency of a capacitor:
                    fr        =                  1                      2            ⁢            π            ⁢                          LC                                                          (        1        )            where fr represents the self-resonance frequency, L represents the parasitic inductance (i.e., equivalent series inductance “ESL”), and C represents the parasitic capacitance (i.e., equivalent series capacitance “ESC”). According to Eq. (1), a capacitor with smaller capacitance may have higher self-resonance frequency, thereby having a broad frequency bandwidth. On the other hand, a capacitor with larger capacitance may have lower self-resonance frequency, thereby having a narrow frequency bandwidth. However, for decoupling capacitors, it is highly desirable to have a high self-resonance frequency and high capacitance.
Capacitance, in general, can be determined by the equation below:
                    C        =                  ɛ          ⁢                                          ⁢                      A            d                                              (        2        )            where C represents the capacitance of the capacitor in Farads, ∈ represents the dielectric constant of the dielectric material, and A represents the surface area of the dielectric material held between two conducting plates and d represents the distance between the plates. According to Eq. (2) above, capacitance is proportional to the surface area of the conducting plates and the dielectric constant, of the dielectric material, and inversely proportional to the distance between the plates. Thus, in order to increase the capacitance of a capacitor, one may increase the area of the conducting plates or select an extremely thin layer of a dielectric material with a high dielectric constant. However, each of these approaches presents difficulties. First, increasing the area of the conducting plates departs from the object of compact designs. In addition, the selection of the dielectric material is often limited by many production and configuration limitations. Additional difficulties arise when the thickness of a dielectric layer is reduced. In particular, the thickness of a dielectric layer on a circuit board can be difficult to control because dielectric thickness may be dramatically changed due to the shapes and dimensions of the patterned features (e.g., capacitor electrodes) over which dielectric is deposited. A thin-dielectric layer design usually comes with the danger of having metal-to-metal shorting through the thin dielectric layer and of having microscopic voids or other structural defects in the layer that may impact capacitive effects and characteristics.